This invention pertains to the evaluation of geological formations from in situ measurements of thermal properties, particularly the determination of formation heat capacity and thermal conductivity from temperature measurements, and the determination of other formation properties--such as porosity, water saturation, and hydrocarbon saturation--from the measured heat capacity and thermal conductivity, or vice versa.
Roux et al., "An Improved Approach to Estimating True Reservoir Temperature from Transient Temperature Data," paper presented at a meeting of the Society of Petroleum Engineers (Los Angeles, Apr. 9-11, 1980), presents a modified Horner-type method for estimating static formation temperature from temperature buildup data, but does not suggest a method for estimating heat conductivity or heat capacity.
Somerton et al., "Some Thermal Characteristics of Porous Rocks," Petroleum Trans. AIME, pp 375-78 (1958), discusses the relationship between thermal conductivity and volumetric heat capacity of rock, and the rock's composition, porosity, and fluid saturation. The derived relationship for volumetric heat capacity will generally be more accurate than that for heat conductivity.
The "Horner" method has been used previously for measuring formation permeability from pressure buildup measurements. See Horner, "Pressure Buildup in Wells," 3d World Petro. Cong. Proc., Sec. II, Preprint 7 pp. 25-43 (1951), which is incorporated by reference.
Earlougher, Advances in Well Test Analysis, pp 45-48 (1977), discusses using a Horner-type method in pressure buildup testing, in which reservoir permeability is estimated from the slope of a Horner plot.
Poppendiek, U.S. Pat. No. 4,343,181, discusses a method for determining the thermal conductivity and heat capacity of the earth, in which a probe heats a limited portion of a formation, and the actual temperature response of the formation is compared to calculated values. At short times, the heat capacity is said to dominate the temperature response curve, and at long times, the thermal conductivity is said to dominate. Because the probe heats only a limited portion of the formation, vertical heat flux affects the measurements. Problems arising from vertical heat flux are not discussed.
Howell, U.S. Pat. No. 3,668,927 discloses a bore hole probe with heating means and temperature measuring means. The slope of a plot of temperature versus logarithm of time is said to be proportional to the thermal conductivity. The method is stated to work either during heating, or during cooling after the heat source is removed. Problems arising from vertical heat flux are not discussed.
Smith, U.S. Pat. Nos. 3,807,227 and 3,892,128, discuss thermal well logging methods for measuring specific heat and thermal conductivity. A single heat source and several thermal detectors are moved through an open or cased bore, and the thermal responses recorded. The specific heat and thermal conductivity are then inferred, and used to make qualitative evaluations of likely locations of water and hydrocarbon deposits. No means for inferring porosity is disclosed. Problems arising from vertical heat flux are not discussed.
Smith, U.S. Pat. No. 3,864,969, has some disclosure cumulative with that of the 3,807,227 and 3,892,128 Patents. This reference discloses a method to determine heat capacity and thermal conductivity by heating one spot within the formation, and then monitoring the temperature relaxation when the heat source is removed. An empirically determined proportionality factor for the specific tool used is determined; there is no disclosure of means for determining heat capacity and thermal conductivity from the temperature measurements alone, without calibration to a specific tool. If the porosity is independently known, an empirical method is given for estimating fluid saturation, content, and type from the measured thermal properties. Problems arising from vertical heat flux are not discussed.
Howell, U.S. Pat. No. 3,981,187, discussed the use of temperature sensors and a single heat source to measure the thermal conductivity of a well casing or a formation. An apparatus comprising the heat source and temperature sensors is moved vertically through the well. The linear portion of a plot of temperature versus logarithm of time is said to have slope Q/k. Heat flux is said to be higher in the vicinity of oil-bearing formations than in the vicinity of non-oil-bearing formations. No means for making quantitative measurements of hydrocarbon saturation is disclosed. Problems arising from vertical heat flux are not discussed. It appears that the apparatus disclosed would not cause heating which is substantially uniform radially, because the heating means contacts only part of the bore hole wall at the depth at which it is located.
Despite the long-felt need for a convenient, inexpensive, easy-to-perform in situ method of measuring formation thermal conductivity, heat capacity, porosity, and water and hydrocarbon saturation levels, both in open and cased wells, at multiple depths, without substantial problems from vertical heat flux, and despite the common availability of apparatus for measuring temperatures inside wells, to the knowledge of the inventors no previous reference has disclosed a practical, working means for making such measurements.